Although the question is quite simple, the answer is not. When H₂ and O₂ react, they don't directly form water. Because the reaction requires such high temperatures, many different reactions can happen simultaneously. Most likely, H₂ and O₂ react to produce 2 OH molecules, which, among many other side- reactions, can reach with another molecule of H₂ to make water.

If 2 hydrogen molecules and 1 oxygen molecules combine, that would give 2 molecules of H₂O (water). That's a balanced equation for the reaction:
2 H₂ + 1 O₂ = 2 H₂O.

Keep asking questions! Best wishes,

Two hydrogen atoms and ONE oxygen atom combine to make water. Two hydrogen atoms and two oxygen atoms together make hydrogen peroxide, which is less stable than water. They'd rather make water.

The way that this happens is that FOUR hydrogen atoms (i.e. TWO hydrogen molecules) combine with two oxygen atoms to make TWO molecules of water.

The second oxygen typically combines with two other hydrogen atoms (different from the first two you mentioned). We usually don't talk about just one molecule of something forming; normally in the physical world we deal with many atoms or molecules at once. For instance, in just one ounce of material there can be over 10²³ atoms or molecules!

That is an excellent question! The second oxygen goes into making a second molecule of water. This is why it is important to balance chemical equations. If we take the equation you said in words and write it with chemical symbols it looks like: H₂ + O₂ -> H₂O and you are right, there is really an extra oxygen that does not show up in the equation. To make the equation balanced, we write it like this:

2H₂ + O_{2<.sub> -> 2H2O
Now there are the same number of each type of atoms on both sides of the equation! It takes twice as much hydrogen gas as oxygen gas to react and make water.}